Reverse alternate levels of a perfect binary tree using Stack

Namachila November 19, 2025

0 1 8 minutes read

Given a Perfect Binary Tree, the task is to reverse the alternate level nodes of the binary tree.
Examples:

Input:
               a
            /     \
           b       c
         /  \     /  \
        d    e    f    g
       / \  / \  / \  / \
       h  i j  k l  m  n  o  
Output:
Inorder Traversal of given tree
h d i b j e k a l f m c n g o 
Inorder Traversal of modified tree
o d n c m e l a k f j b i g h

Input:
               a
              / \
             b   c
Output:
Inorder Traversal of given tree
b a c 
Inorder Traversal of modified tree
c a b

Approach: Another approach to the problem is discussed here. In this article, we discuss an approach involving stack.
Traverse the tree in a depth-first fashion and for each level,

If the level is odd, push the left and right child(if exists) in a stack.
If the level is even, replace the value of the current node with the top of the stack.

Below is the implementation of the above approach:

C++

// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;
 
// A tree node
struct Node {
    char key;
    struct Node *left, *right;
};
 
// Utility function to create new Node
Node* newNode(int key)
{
    Node* temp = new Node;
    temp->key = key;
    temp->left = temp->right = NULL;
    return (temp);
}
 
// Utility function to perform
// inorder traversal of the tree
void inorder(Node* root)
{
    if (root != NULL) {
        inorder(root->left);
        cout << root->key << " ";
        inorder(root->right);
    }
}
 
// Function to reverse alternate nodes
void reverseAlternate(Node* root)
{
 
    // Queue for depth first traversal
    queue<Node*> q;
    q.push(root);
    Node* temp;
 
    // Level of root considered to be 1
    int n, level = 1;
 
    // Stack to store nodes of a level
    stack<int> s;
 
    while (!q.empty()) {
        n = q.size();
        while (n--) {
            temp = q.front();
            q.pop();
 
            // If level is odd
            if (level % 2) {
 
                // Store the left and right child
                // in the stack
                if (temp->left) {
                    q.push(temp->left);
                    s.push(temp->left->key);
                }
 
                if (temp->right) {
                    q.push(temp->right);
                    s.push(temp->right->key);
                }
            }
 
            // If level is even
            else {
 
                // Replace the value of node
                // with top of the stack
                temp->key = s.top();
                s.pop();
 
                if (temp->left)
                    q.push(temp->left);
                if (temp->right)
                    q.push(temp->right);
            }
        }
 
        // Increment the level
        level++;
    }
}
 
// Driver code
int main()
{
    struct Node* root = newNode('a');
    root->left = newNode('b');
    root->right = newNode('c');
    root->left->left = newNode('d');
    root->left->right = newNode('e');
    root->right->left = newNode('f');
    root->right->right = newNode('g');
    root->left->left->left = newNode('h');
    root->left->left->right = newNode('i');
    root->left->right->left = newNode('j');
    root->left->right->right = newNode('k');
    root->right->left->left = newNode('l');
    root->right->left->right = newNode('m');
    root->right->right->left = newNode('n');
    root->right->right->right = newNode('o');
 
    cout << "Inorder Traversal of given tree\n";
    inorder(root);
 
    reverseAlternate(root);
 
    cout << "\nInorder Traversal of modified tree\n";
    inorder(root);
 
    return 0;
}

Java

// Java implementation of the approach 
import java.util.*;
 
class GfG
{ 
 
// A tree node 
static class Node
{ 
    char key; 
    Node left, right; 
}
 
// Utility function to create new Node 
static Node newNode(char key) 
{ 
    Node temp = new Node(); 
    temp.key = key; 
    temp.left = temp.right = null; 
    return (temp); 
} 
 
// Utility function to perform 
// inorder traversal of the tree 
static void inorder(Node root) 
{ 
    if (root != null)
    { 
        inorder(root.left); 
        System.out.print(root.key + " "); 
        inorder(root.right); 
    } 
} 
 
// Function to reverse alternate nodes 
static void reverseAlternate(Node root) 
{ 
 
    // Queue for depth first traversal 
    Queue<Node> q = new LinkedList<Node> (); 
    q.add(root); 
    Node temp; 
 
    // Level of root considered to be 1 
    int n, level = 1; 
 
    // Stack to store nodes of a level 
    Stack<Character> s = new Stack<Character> (); 
 
    while (!q.isEmpty())
    { 
        n = q.size(); 
        while (n != 0)
        { 
            if(!q.isEmpty()) 
            {
                temp = q.peek(); 
                q.remove(); 
            }
            else
            temp = null;
             
            // If level is odd 
            if (level % 2 != 0)
            { 
 
                // Store the left and right child 
                // in the stack 
                if (temp != null && temp.left != null) 
                { 
                    q.add(temp.left); 
                    s.push(temp.left.key); 
                } 
 
                if (temp != null && temp.right != null)
                { 
                    q.add(temp.right); 
                    s.push(temp.right.key); 
                } 
            } 
 
            // If level is even 
            else
            { 
 
                // Replace the value of node 
                // with top of the stack 
                temp.key = s.peek(); 
                s.pop(); 
 
                if (temp.left != null) 
                    q.add(temp.left); 
                if (temp.right != null) 
                    q.add(temp.right); 
            }
            n--;
        } 
 
        // Increment the level 
        level++; 
    } 
} 
 
// Driver code 
public static void main(String[] args) 
{ 
    Node root = newNode('a'); 
    root.left = newNode('b'); 
    root.right = newNode('c'); 
    root.left.left = newNode('d'); 
    root.left.right = newNode('e'); 
    root.right.left = newNode('f'); 
    root.right.right = newNode('g'); 
    root.left.left.left = newNode('h'); 
    root.left.left.right = newNode('i'); 
    root.left.right.left = newNode('j'); 
    root.left.right.right = newNode('k'); 
    root.right.left.left = newNode('l'); 
    root.right.left.right = newNode('m'); 
    root.right.right.left = newNode('n'); 
    root.right.right.right = newNode('o'); 
 
    System.out.println("Inorder Traversal of given tree"); 
    inorder(root); 
 
    reverseAlternate(root); 
 
    System.out.println("\nInorder Traversal of modified tree"); 
    inorder(root); 
}
} 
 
// This code is contributed by Prerna Saini

Python3

# Python3 implementation of the 
# above approach
 
# A tree node
class Node:
     
    def __init__(self, key):
       
        self.key = key
        self.left = None
        self.right = None
     
# Utility function to
# create new Node
def newNode(key):
 
    temp = Node(key)
    return temp
 
# Utility function to perform
# inorder traversal of the tree
def inorder(root):
 
    if (root != None):
        inorder(root.left);
        print(root.key,
              end = ' ')
        inorder(root.right);    
 
# Function to reverse 
# alternate nodes
def reverseAlternate(root):
  
    # Queue for depth 
    # first traversal
    q = []
    q.append(root);
     
    temp = None
  
    # Level of root considered 
    # to be 1
    n = 0
    level = 1;
  
    # Stack to store nodes 
    # of a level
    s = []
  
    while (len(q) != 0):        
        n = len(q);        
        while (n != 0):            
            n -= 1
            temp = q[0];
            q.pop(0);
  
            # If level is odd
            if (level % 2 != 0):
  
                # Store the left and 
                # right child in the stack
                if (temp.left != None):
                    q.append(temp.left);
                    s.append(temp.left.key);
                 
  
                if (temp.right != None):
                    q.append(temp.right);
                    s.append(temp.right.key);
  
            # If level is even
            else:
  
                # Replace the value of node
                # with top of the stack
                temp.key = s[-1];
                s.pop();
  
                if (temp.left != None):
                    q.append(temp.left);
                if (temp.right != None):
                    q.append(temp.right);
  
        # Increment the level
        level += 1;    
 
# Driver code
if __name__ == "__main__":
     
    root = newNode('a');
    root.left = newNode('b');
    root.right = newNode('c');
    root.left.left = newNode('d');
    root.left.right = newNode('e');
    root.right.left = newNode('f');
    root.right.right = newNode('g');
    root.left.left.left = newNode('h');
    root.left.left.right = newNode('i');
    root.left.right.left = newNode('j');
    root.left.right.right = newNode('k');
    root.right.left.left = newNode('l');
    root.right.left.right = newNode('m');
    root.right.right.left = newNode('n');
    root.right.right.right = newNode('o');
     
    print("Inorder Traversal of given tree")
    inorder(root);
  
    reverseAlternate(root);
     
    print("\nInorder Traversal of modified tree")
    inorder(root);
     
# This code is contributed by Rutvik_56

C#

// C# implementation of the approach 
using System;
using System.Collections;
 
class GfG 
{ 
 
// A tree node 
public class Node 
{ 
    public char key; 
    public Node left, right; 
} 
 
// Utility function to create new Node 
static Node newNode(char key) 
{ 
    Node temp = new Node(); 
    temp.key = key; 
    temp.left = temp.right = null; 
    return (temp); 
} 
 
// Utility function to perform 
// inorder traversal of the tree 
static void inorder(Node root) 
{ 
    if (root != null) 
    { 
        inorder(root.left); 
        Console.Write(root.key + " "); 
        inorder(root.right); 
    } 
} 
 
// Function to reverse alternate nodes 
static void reverseAlternate(Node root) 
{ 
 
    // Queue for depth first traversal 
    Queue q = new Queue (); 
    q.Enqueue(root); 
    Node temp; 
 
    // Level of root considered to be 1 
    int n, level = 1; 
 
    // Stack to store nodes of a level 
    Stack s = new Stack (); 
 
    while (q.Count > 0) 
    { 
        n = q.Count; 
        while (n != 0) 
        { 
            if(q.Count > 0) 
            { 
                temp = (Node)q.Peek(); 
                q.Dequeue(); 
            } 
            else
            temp = null; 
             
            // If level is odd 
            if (level % 2 != 0) 
            { 
 
                // Store the left and right child 
                // in the stack 
                if (temp != null && temp.left != null) 
                { 
                    q.Enqueue(temp.left); 
                    s.Push(temp.left.key); 
                } 
 
                if (temp != null && temp.right != null) 
                { 
                    q.Enqueue(temp.right); 
                    s.Push(temp.right.key); 
                } 
            } 
 
            // If level is even 
            else
            { 
 
                // Replace the value of node 
                // with top of the stack 
                temp.key =(char)s.Peek(); 
                s.Pop(); 
 
                if (temp.left != null) 
                    q.Enqueue(temp.left); 
                if (temp.right != null) 
                    q.Enqueue(temp.right); 
            } 
            n--; 
        } 
 
        // Increment the level 
        level++; 
    } 
} 
 
// Driver code 
public static void Main(String []args) 
{ 
    Node root = newNode('a'); 
    root.left = newNode('b'); 
    root.right = newNode('c'); 
    root.left.left = newNode('d'); 
    root.left.right = newNode('e'); 
    root.right.left = newNode('f'); 
    root.right.right = newNode('g'); 
    root.left.left.left = newNode('h'); 
    root.left.left.right = newNode('i'); 
    root.left.right.left = newNode('j'); 
    root.left.right.right = newNode('k'); 
    root.right.left.left = newNode('l'); 
    root.right.left.right = newNode('m'); 
    root.right.right.left = newNode('n'); 
    root.right.right.right = newNode('o'); 
 
    Console.WriteLine("Inorder Traversal of given tree"); 
    inorder(root); 
 
    reverseAlternate(root); 
 
    Console.WriteLine("\nInorder Traversal of modified tree"); 
    inorder(root); 
} 
} 
 
// This code is contributed by Arnab Kundu

Javascript

<script>
 
// JavaScript implementation of the approach 
 
// A tree node 
class Node 
{ 
    constructor()
    {
        this.key = '';
        this.left = null;
        this.right = null;
    }
} 
 
// Utility function to create new Node 
function newNode(key) 
{ 
    var temp = new Node(); 
    temp.key = key; 
    temp.left = temp.right = null; 
    return (temp); 
} 
 
// Utility function to perform 
// inorder traversal of the tree 
function inorder(root) 
{ 
    if (root != null) 
    { 
        inorder(root.left); 
        document.write(root.key + " "); 
        inorder(root.right); 
    } 
} 
 
// Function to reverse alternate nodes 
function reverseAlternate(root) 
{ 
 
    // Queue for depth first traversal 
    var q = []; 
    q.push(root); 
    var temp = null; 
 
    // Level of root considered to be 1 
    var n, level = 1; 
 
    // Stack to store nodes of a level 
    var s = [];
 
    while (q.length > 0) 
    { 
        n = q.length; 
        while (n != 0) 
        { 
            if(q.length > 0) 
            { 
                temp = q[0]; 
                q.shift(); 
            } 
            else
            temp = null; 
             
            // If level is odd 
            if (level % 2 != 0) 
            { 
 
                // Store the left and right child 
                // in the stack 
                if (temp != null && temp.left != null) 
                { 
                    q.push(temp.left); 
                    s.push(temp.left.key); 
                } 
 
                if (temp != null && temp.right != null) 
                { 
                    q.push(temp.right); 
                    s.push(temp.right.key); 
                } 
            } 
 
            // If level is even 
            else
            { 
 
                // Replace the value of node 
                // with top of the stack 
                temp.key = s[s.length-1]; 
                s.pop(); 
 
                if (temp.left != null) 
                    q.push(temp.left); 
                if (temp.right != null) 
                    q.push(temp.right); 
            } 
            n--; 
        } 
 
        // Increment the level 
        level++; 
    } 
} 
 
// Driver code 
var root = newNode('a'); 
root.left = newNode('b'); 
root.right = newNode('c'); 
root.left.left = newNode('d'); 
root.left.right = newNode('e'); 
root.right.left = newNode('f'); 
root.right.right = newNode('g'); 
root.left.left.left = newNode('h'); 
root.left.left.right = newNode('i'); 
root.left.right.left = newNode('j'); 
root.left.right.right = newNode('k'); 
root.right.left.left = newNode('l'); 
root.right.left.right = newNode('m'); 
root.right.right.left = newNode('n'); 
root.right.right.right = newNode('o'); 
document.write("Inorder Traversal of given tree<br>"); 
inorder(root); 
reverseAlternate(root); 
document.write("<br>Inorder Traversal of modified tree<br>"); 
inorder(root); 
 
</script>

Output:

Inorder Traversal of given tree
h d i b j e k a l f m c n g o 
Inorder Traversal of modified tree
o d n c m e l a k f j b i g h

The time complexity of the above approach is O(N) where N is the number of nodes in the tree as we are traversing through all the nodes once.

The space complexity of the above approach is O(N) as we are using a queue and a stack which can store up to N elements.

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